Differentials have been used in the drive systems of wheeled vehicles for many years, to allow the wheels on opposite sides of the vehicle being driven by a common drive shaft to rotate at different speeds while the vehicle is making a turn, so that the outer wheel is not forced to skid on the driving surface as the vehicle negotiates the turn. Differentials also allow wheels of different diameters to be driven by a common drive shaft without skidding one or the other of the wheels, so that a vehicle may be operated with a partially deflated tire, or with a compact spare.
In a typical differential, the wheels on either side of the vehicle are driven by separate axle shafts joined at an inner end by the differential. The inner end of each axle engages its own side gear, and the side gears are connected together by planet gears mounted inside of a rotating differential case, that is in turn mounted in a non-rotating differential housing. The differential case is connected through a ring and pinion gear arrangement to be rotated by a drive shaft connected to the vehicle engine through a transmission or similar drive train component.
This gearing arrangement allows the wheels to turn at different speeds with respect to one another, while being driven by the drive shaft, which is turning at a constant speed. The sum of the speeds of the wheels remains constant, so that as the inside wheel slows down during a turn and the outside wheel speeds up, due to the difference in the distance that the wheels must travel to traversing the turn, the sum of the speeds of the wheels will remain a constant value. If one of the wheels should start to slip, or spin, however, the differential will allow the other wheel to slow down by a proportionate amount, or even stop altogether with the slipping wheel spinning at twice the speed it would be turning if both wheels were driving the vehicle straight ahead without slipping. In such circumstances, traction is greatly reduced or lost altogether, and the vehicle can become stuck.
To address this problem, a special type of differentials, known as limited slip differentials have been utilized for many years in wheeled vehicles for controlling the degree to which the wheels on opposite sides of the vehicle can vary from one another. A typical limited slip differential includes a hydraulically actuated friction clutch pack that is actuated to retard relative rotation of at least one of the side gears of the differential, relative to the rotation of a differential case in which the side gears are mounted. Normally, hydraulic pressure for actuating the clutch pack is generated within the differential, in a manner that results in pressure being increased or decreased in direct proportion to an increase or decrease in the speed of the side gears with respect to the differential case.
To provide the pressure for actuating the clutch pack, a limited slip differential often includes a gerotor pump having an inner gear attached to one of the side gears, and an eccentrically mounted outer gear rotating within the differential case. Whenever the side gear is rotating, the pump generates a pressurized flow of hydraulic fluid to a piston chamber in the case having a piston that clamps clutch disks together in the clutch pack to retard relative rotation between the side gear and the case.
Historically, pressure in the piston chamber, and clamping force generated by the piston, have been controlled with a spring biased pressure relief valve mounted in the differential case and a flow-controlling orifice. The relief valve and orifice are traditionally placed in a series fluid circuit relationship to one another in a fluid passage providing an outlet for fluid in the piston chamber.
When hydraulic pressure reaches a predetermined value, the pressure relief valve opens and begins to release a portion of the fluid being pumped into the piston chamber by the pump, through the flow-controlling orifice. The fluid released by the relief valve is preferrably collected in a non-rotating plenum that directs the fluid back to the pump inlet for re-circulation through the piston chamber. As the fluid passes through the relief valve and flow-controlling orifice, the pressure drops from a high pressure inside the piston chamber to a low pressure in the plenum, thus allowing the plenum to be sealed to the differential case with low pressure dynamic seals. U.S. Pat. No. 6,283,885 B1 to Irwin, U.S. Pat. No. 6,342,022 B1 to Sturm, and U.S. Pat. No. 6,413,182 B1 to Yates III, et al, illustrate this approach.
While using a relief valve in this manner works well, this approach suffers from a disadvantage in that the hydraulic pressure release characteristic of the relief valve is fixed by the valve spring rate and component design, and cannot be varied while the differential is operating. In order to change the relief valve setting, the differential must be disassembled so that the relief valve components can be changed.
In modem vehicles having sophisticated controlled braking systems for improving traction and stability of the vehicle, it is highly desirable to have the capability for totally disengaging and/or changing the operating characteristics of the limited slip differential during operation of the vehicle.
In one approach to providing a capability for disengaging and/or changing the operating characteristics of the limited slip differential during operation of the vehicle, an actuating mechanism located inside or outside of the differential housing is connected to a relief valve of the type described above, for adjusting the spring rate of the valve during operation of the vehicle. The actuating mechanism may be driven electrically, or by other mechanical, hydraulic, pneumatic or vacuum powered actuators. An example of this approach using an actuating mechanism having a fork shift mechanism is disclosed in U.S. Pat. No. 6,183,387 B1 to Yoshioka. Using an actuating mechanism, such as the one disclosed in Yoshioka, is undesirable, however, in that it requires a complex and bulky linkage, and does not include a plenum for efficient control of the fluid released by the relief valve.
In other approaches, the relief valve inside the differential housing is eliminated through the use of a solenoid operated flow control valve. An example of this approach is provided by a commonly assigned US patent application Ser. No. 10/309,219 to Schrand, et al,. In some embodiments of this approach, a plenum includes a high pressure cavity connected via an open fluid passage to the piston chamber, and sealed to the differential case with high pressure dynamic seals. The solenoid operated valve is installed in the plenum at an outlet of the high pressure cavity and releases fluid into the interior of the differential housing.
While using a solenoid operated valve located on the plenum, rather than on the differential case, offers considerable improvement over the approach disclosed in Yoshioka, and generally works well, it would be desirable to eliminate the need for high pressure seals between the plenum and the differential case. Having the solenoid operated valve located on the plenum also requires that the differential housing be larger to provide room for the solenoid portion of the valve.
What is needed, therefore, is an improved apparatus and method for controlling a limited slip differential.